Recyclable carbon supported copper-manganese oxide for selective aerobic oxidation of alcohols in combination with 2,2,6,6-tetramethylpiperidyl-1-oxyl under neutral condition

Article abstract of DOI:10.1002/adsc.200700513

Due to the promotion of the surface area and the dispersion of active components upon supporting mixed metal oxides on the porous material active carbon, the copper-manganese oxide on carbon system has been proven to be much more efficient than the co-precipitation prepared Cu-Mn oxide in mediating the 2,2,6,6-tetramethylpiperidyl-1-oxyl (TEMPO)-catalyzed aerobic oxidation of alcohols. Even at 30°C and with a 0.1 mol% load of TEMPO, the oxidations proceeded smoothly. Upon catalysis with the Cu-Mn oxide/C (10 wt%) and TEMPO (0.5-5 mol%), various alcohols were oxidized selectively to the corresponding aldehydes or ketones with molecular oxygen at 80°C. Such a stable, recyclable heterogeneous cocatalyst permits alcohols to be oxidized under neutral and mild conditions.

Full text of DOI:10.1002/adsc.200700513

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DOI: 10.1002/adsc.200700513
Recyclable Carbon Supported Copper-Manganese Oxide for
Selective Aerobic Oxidation of Alcohols in Combination with
2,2,6,6-Tetramethylpiperidyl-1-oxyl under Neutral Condition
a,
a
a
a
a
Guanyu Yang, * Weimin Zhu, Panke Zhang, Huazhen Xue, Wei Wang,
a
a,
Junshan Tian, and Maoping Song *
a
Department ofChemistry, Zhengzhou University, Zhengzhou 450052, Peopleꢀs Republic ofChina
Fax : (+86)-371-6776-3927; e-mail: yangguanyu@zzu.edu.cn
Received: October 27, 2007; Revised: January 5, 2008; Published online: February 26, 2008
Supporting information for this article is available on the WWW under http://asc.wiley-vch.de/home/.
[
9]
[10]
Abstract: Due to the promotion ofthe sur af ce area
and the dispersion ofactive components upon sup-
porting mixed metal oxides on the porous material
active carbon, the copper-manganese oxide on
carbon system has been proven to be much more
efficient than the co-precipitation prepared Cu-Mn
oxide in mediating the 2,2,6,6-tetramethylpiperidyl-
in acetic acid,
CuBr (2,2’-bipyridine)/t-BuOK,
2
[
11]
[12]
RuCl
A
H
R
U
(PPh ) ,
CuBr·Me S,
CH ReO /HBr in
2
3
[
3
2
3 3
13]
[14]
acetic acid, Cu(II)-diimine complexes, etc. These
multicomponent cocatalytic systems build one (or
more) redox bridge(s) between molecular oxygen ac-
tivation and in situ oxidation ofTEMPO to its oxam-
monium cation, which is the real oxidant for the se-
lective oxidation ofalcohols. However, these homoge-
neous cocatalysts are difficult to recover and are
often discarded in the waste water; also the acidic re-
action conditions and the use ofhalides tend toward
potential equipment corrosion in industrial applica-
tions.
1-oxyl (TEMPO)-catalyzed aerobic oxidation ofal-
cohols. Even at 308C and with a 0.1 mol% load of
TEMPO, the oxidations proceeded smoothly. Upon
catalysis with the Cu-Mn oxide/C (10 wt%) and
TEMPO (0.5–5 mol%), various alcohols were oxi-
dized selectively to the corresponding aldehydes or
ketones with molecular oxygen at 808C. Such a
stable, recyclable heterogeneous cocatalyst permits
alcohols to be oxidized under neutral and mild con-
ditions.
Growing attention is being given to developing het-
erogeneous catalysts for alcohol oxidation because of
their simple separability and recyclability, which are
[
15]
ofsign ii fc ant industrial interest.
Based on the as-
sumption that the redox-active heterogeneous materi-
al could promote the transformation of TEMPO to its
oxammonium cation, we found that co-precipitation
prepared Cu-Mn mixed oxides (the molar ratio ofCu
and Mn over 1) are recyclable heterogeneous cocata-
lysts for TEMPO-catalyzed aerobic oxidation of alco-
Keywords: aerobic oxidation; alcohols; carbon; cat-
alysis; copper-manganese mixed oxide; 2,2,6,6-tetra-
methylpiperidyl-1-oxyl (TEMPO)
[
16]
hols under neutral condition, avoiding the usage of
acidic solvents and halogen-containing cocatalyst. In
The transformation of alcohols to the corresponding continuation ofour study on this subject with consid-
aldehydes or ketones is ofimportance in fu ndamental eration ofthe fa ct that the porous material could pro-
[
1]
research and industrial manufacturing. Many cata- mote the surface area and the dispersion of active
lytic systems have been discovered for using molecu- components, as well as the activity ofcatalyst, we are
lar oxygen as environmentally friendly oxidant for the particularly interested in exploring the potential of
[
2]
selective oxidation ofalcohols. Ofparticular interest
the supported Cu-Mn oxide. Herein, we report on the
are the catalytic systems using 2,2,6,6-tetramethylpi- aerobic oxidation ofalcohols by a coupled system of
[
3]
peridyl-1-oxyl (TEMPO) as a key catalyst in combi- the Cu-Mn mixed oxide supported on active carbon
nation with various homogeneous multicomponent and TEMPO.
[
4]
cocatalytic systems, such as, tert-butyl nitrite/HBr,
The Cu-Mn oxide/C catalyst was prepared using the
[5]
[6]
NaNO /FeCl ,
NaNO /Br ,
2 2
[7]
NaNO /1,3-dibromo- incipient wetness method according to the litera-
(acetoxy)iodo]benzene/ ture.
2
3
2
[17]
5
,5-dimethylhydantoin,
[bis
A
H
R
U
G
The commercial active carbon (5.0 g, 60–80
[8]
À1
KNO2, Mn
A
C
H
T
R
E
U
N
G
(NO ) /Co
A
H
R
U
G
A
H
E
N
(NO ) /Cu
A
T
U
mesh, sorption capacity 1.4 mLg ) was impregnated
3
2
3
2
3
2
542
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Synth. Catal. 2008, 350, 542 – 546

Recyclable Carbon Supported Copper-Manganese Oxide for Selective Aerobic Oxidation
COMMUNICATIONS
with an aqueous solution ofcopper nitrate (3 mmol)
ture increased the reaction rate, and 808C seemed to
and manganese nitrate (1 mmol), and then was dried be the preferable temperature. The pressure of molec-
at 1158C. Finally, the Cu-Mn oxide/C catalyst was ob- ular oxygen shown a positive influence on the rate
tained by calcination in a vacuum quartz tube at below 0.6 MPa, but above 0.6 MPa it lost its effect.
4
008C in flowing N for 4 h.
This suggests that the rate ofoxidation is controlled
2
In the initial tests on the aerobic oxidation of by molecular oxygen diffusing inside the Cu-Mn
benzyl alcohol, it was encouraging that the carbon oxide/C under the lower presure. However, under
supported Cu-Mn oxide (10 wt%) in combination 0.4 MPa at 808C the oxidation ofbenzyl alcohol to
with TEMPO (5 mol%) gave 90.9% conversion of benzaldehyde still reached a near quantitative result
benzyl alcohol with > 99% selectivity for benzalde- on prolonging the reaction time to 12 h (entry 13,
hyde at 808C under 0.4 MPa O2 for 6 h (entry 7, Table 1). So 0.4 MPa ofmolecular oxygen was chosen
Table 1), and thus exhibited a much better catalytic in the further optimal test. It was not unexpected to
performance than the coprecipitation prepared Cu- find that the more loading amounts of both the Cu-
Mn mixed oxide with the same molar ratio ofCu and
Mn oxide/C and TEMPO result in faster rates of oxi-
Mn which realized merely 31.4% conversion under dation (entries 14–20, Table 1). However, the influ-
the same conditions. On the other hand, no peak of ence ofthe loading amount ofthe Cu-Mn oxide/C on
active components was found in the XRD pattern of the rate was obviously stronger than that ofTEMPO.
the Cu-Mn oxide/C catalyst (see Supporting Informa- In the course of6 h, 1 mol% TEMPO and 10 wt%
tion). This implied, as expected, that the catalytic ac- Cu-Mn oxide/C realized 98.9% conversion (entry 19,
tivity ofthe Cu-Mn mixed oxide could be promoted
Table 1), but 1 wt% Cu-Mn oxide/C and 5 mol%
by supporting it on a porous material due to the in- TEMPO gave only 14.2% conversion (entry 16,
crease ofthe sur af ce area ofthe catalyst and the dis-
persion ofthe active components.
Table 1). These mean that the oxidation rate depends
on the reactants diffusing to the catalytic sites on the
The optimal catalytic results for the benzyl alcohol Cu-Mn oxide/C because the oxidation occurs on the
oxidation using the coupled system ofthe Cu-Mn intrasurface of solid catalyst. Decreasing the load
oxide/C and TEMPO are displayed in Table 1. As can amount ofTEMPO to 0.5 mol% and 0.1 mol% in
be seen, the coupled system can work even at as low combination with 10 wt% Cu-Mn oxide/C, the oxida-
as 308C, giving ca. 54% conversions in the course of tion realized 80.1% and 48.2% conversion, respective-
6
h (entry 1 and 2, Table 1). Increasing the tempera- ly (entry 21 and 22, Table 1). Prolonging the time to
[a]
Table 1. Aerobic oxidation ofbenzyl alcohol to benzaldehyde by the Cu-Mn oxide/C and TEMPO.
o
Entry
Cu-Mn oxide/C [wt%]
TEMPO [mol%]
T [ C]
P [MPa]
Time [h]
Conversion [%]
Selectivity [%]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
10
10
10
10
10
10
10
10
10
10
10
10
10
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
5
3
3
1
1
0.5
0.1
0.5
0
30
30
40
60
80
100
80
80
80
40
80
80
80
80
80
80
80
80
80
80
80
80
80
80
0.4
0.8
0.8
0.8
0.8
0.8
0.4
0.6
1.0
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
6
6
6
6
6
6
6
6
6
18
3
6
12
6
6
6
6
3
54.6
54.2
64.8
87.3
98.8
99.0
90.9
98.8
98.8
82.6
81.3
90.9
99.0
41.5
27.4
14.2
100
87.6
98.9
79.5
80.1
48.2
97.9
1.6
90.4
92.9
93.9
>99
>99
>99
>99
>99
>99
93.0
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
-
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
3
1
10
10
10
10
10
10
10
10
6
3
6
8
24
6
[
a]
The oxidations were carried out using 2 mL benzyl alcohol and 10 mL CH Cl in all cases.
2
2
Adv. Synth. Catal. 2008, 350, 542 – 546
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asc.wiley-vch.de
543

Guanyu Yang et al.
COMMUNICATIONS
2
0
4 h increased the conversion to 97.7% in the case of Cu-Mn oxide/C proceeded according to what we had
.5 mol% TEMPO (entry 23, Table 1) and 96.1% iso- documented before.
[
16]
lated yield was obtained. These showed that the low
load ofTEMPO still has signi if cant catalytic activity.
The recycle tests were conducted in order to inves-
tigate the recyclability ofthe Cu-Mn oxide/C. A tf er
As can be seen in Table 1, the selectivities for ben- the reaction, the Cu-Mn oxide/C was filtered and
zaldehyde were all above 99% when the reaction tem- washed with ethanol several times, dried at 1008C f or
peratures were above 608C, but it was surprising that 4 h to remove the ethanol, and then used together
below 608C the selectivities were out ofline, and a
small amount ofbenzoic acid always existed in the
with TEMPO for another reaction. After five runs,
both ofthe conversions and the selectivities remained
oxidations (entries 1–3 and 10, Table 1). It is known at similar levels to that ofthe if rst run. The Cu-Mn
that benzaldehyde could be oxidized easily to benzoic oxide/C without any detectable loss ofactivity in the
acid by molecular oxygen in long-term storage follow- recycle tests thus proves to be stable during catalysis,
ing the radical mechanism, and aldehydes are often and recyclable.
[
18]
used as the radical initiator. Therefore, the contrast
tests were conducted using benzaldehyde as the sub- lyze the aerobic oxidation ofvarious alcohols
strate instead ofbenzyl alcohol. Employing the Cu- (Table 2). It is a general phenomenon that the oxida-
The coupled system was employed further to cata-
Mn oxide/C (10 wt%) alone, benzaldehyde was con- tions ofsecondary alcohols and aliphatic alcohols pro-
verted completely to benzoic acid under 0.4 MPa O2 ceed slowly when Cu compounds and TEMPO are
[
14,19]
within 6 h at 308C, 408C and 608C, respectively. This used as catalysts.
The combination ofthe Cu-Mn
shows us that the Cu-Mn oxide/C has the ability to oxide/C and TEMPO cannot overcome this hurdle. In
catalyze a one-electron oxidation. Hence, we suggest- the presence of10 wt% Cu-Mn oxide/C and 5 mol%
ed that two competitive reactions [see the reactions TEMPO under 0.4 MPa at 808C for 8–10 h, the oxida-
ofEq. (1) and Eq. (2)] occurred on the Cu-Mn oxide/
tions of1-phenethanol, 2-phenethanol, 1-heptanol
and 2-heptanol gave 43.3%, 12.7%, 23.8% and 12.5%
conversions, respectively (entries 9–12, Table 2). How-
ever, the oxidations ofcinnamyl alcohol and all pri-
mary benzylic alcohols with either electron-donating
or electron-withdrawing substituents can be catalyzed
efficiently, among which (4-nitrophenyl)methanol be-
haves almost perfectly, realizing 100% conversion and
99.0% isolated yield. Even with as low as 0.5 mol%
load ofTEMPO, these alcohols can be oxidized
nearly completely to their corresponding aldehydes
by merely prolonging the reaction time.
In summary, we have found that the supported Cu-
Mn oxide/C system is a heterogeneous cocatalyst that
C. The oxidation ofTEMPO to its oxammonium
cation [Eq. (2)] takes place more easily than the aero- can activate directly molecular oxygen and transfer its
bic oxidation ofbenzaldehyde [Eq. (1)]. This is why
all oxidations ofbenzaldehyde using the Cu-Mn
oxidizing power to the oxidation ofTEMPO. As a
result, the coupled catalytic system ofCu-Mn oxide/C
oxide/C (10 wt%) and TEMPO (5 mol%) together and TEMPO has been developed for the aerobic se-
gave as low as ca. 6% conversions under 0.4 MPa O2 lective oxidation ofvarious alcohols to the corre-
for 6 h at temperatures of 308C and 408C, respective- sponding aldehydes or ketones. It works efficiently at
ly. Above 608C the reaction ofEq. (2) becomes so
808C, and the TEMPO load can be decreased to as
fast that Eq. (1) has little chance to ocurr, so benzal- low as 0.5 mol%. The easy handling, simple separa-
dehyde is the sole product in catalytic oxidation of tion and reusability ofsuch a heterogeneous cocata-
benzyl alcohol.
lyst enable the oxidation to be conducted under neu-
Nitrites are often used as a component of the cata- tral condition, and is ofsigni if cant industrial interest.
lytic system in the TEMPO-catalyzed aerobic oxida-
tion ofalcohols, which activate molecular oxygen and
transfer its oxidizability to next component or
[5–8]
Experimental Section
TEMPO.
However, addition ofNaNO 2 or NaNO2/
acetic acid into the system ofthe Cu-Mn oxide/C and
TEMPO did not afford any evident improvement in
the catalytic activity. It could be that molecular
oxygen oxidized directly a metal species oflower va-
General Remarks
Gas chromatography measurements were conducted using
an Agilent Technologies 6890N Network GC System with a
lence to a higher one after the reaction of Eq. (2) had flame ionization detector and a DB-1 capillary column
occurred, and then the constructed redox cycle ofthe (30 m”0.535 mm”3.0 mm). Agilent Chemstation software
544
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Adv. Synth. Catal. 2008, 350, 542 – 546

Recyclable Carbon Supported Copper-Manganese Oxide for Selective Aerobic Oxidation
COMMUNICATIONS
[a]
Table 2. Aerobic oxidations ofvarious alcohols by the Cu-Mn oxide/C and TEMPO.
Entry
1
Substrate
TEMPO [mol%]
5
Time [h]
Conversion [%]
Product
Selectivity [%]
1
24
93.1
100
>99
>99
0.5
2
3
4
3
0
3
16
97.7
97.9
>99
>99
.5
3
0
3
16
87.7
82.4
>99
>99
.5
.5
.5
5
3
0
2.5
2.5
24
96.9
86.0
100
>99
>99
>99
5
5
2
100
>99
3
0
5
3
2
100
100
83.8
75.4
>99
>99
>99
>99
12
3.5
3.5
6
7
8
3
0
3
16
98.7
94.6
>99
>99
.5
5
3
2
2
97.7
97.7
>99
>99
9
5
5
8
8
43.3
12.7
>99
>99
10
1
1
1
2
1-heptanol
2-heptanol
5
5
10
10
23.8
12.5
heptanal
2-heptanone
>99
>99
[
a]
The reactions were carried out under 0.4 MPa ofO 2 at 808C in 10 mL of CH Cl ; the used amounts ofsolid substrates
2
2
were 2 g and those for liquid substrates were 2 mL; 10 wt% of the Cu-Mn oxide/C were used in all case. The conversions
and the selectivities are based on GC with area normalization.
was used for spectra acquisition and processing. The conver-
sions and the selectivities were determined by GC area nor-
malization. All starting materials and catalysts were pur-
chased from commercial sources and used without further
treatment.
after the GC measurement, the ethanol solution of the reac-
tion mixture was removed under reduced pressure, and then
the residue was purified by silica-gel column chromatogra-
phy, EtOAc/hexane (v:v=1:2) used as eluant to afford the
corresponding aldehyde product.
General Typical Procedure for the Oxidation
The reaction was carried out in a 70-mL autoclave and the
general procedure is described typically with benzyl alcohol
as follows: a mixture of TEMPO (151 mg, 5 mol%), Cu-Mn
oxide/C (10 wt%), benzyl alcohol (2 mL, 19.3 mmol), and di-
chloromethane (10 mL) was stirred and heated to 808C
after the atmosphere over the mixture was replaced with O2
Acknowledgements
This work was supported by China Postdoctoral Science
Foundation (No. 20070410251).
for three times. The pressure of O was kept at 0.4 MPa for
2
the desired reaction time. After the reaction mixture had References
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solution was analyzed by gas chromatography.
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